Heparin-induced thrombocytopenia (HIT) is a drug-induced blood disorder that affects 1-5% of patients who are treated with the blood-thinning drug heparin (Chong, 1995, W British Journal of Haematology, 89(3):431-9). Patients who develop HIT are at risk for developing life-threatening clots in the heart, the brain, the extremities or the lungs (Arepally et al., 1998, Clinical Reviews in Allergy and Immunology, 16:1-11). HIT is caused by antibodies which bind specifically with a complex comprising the protein Platelet Factor 4 (PF4) and heparin, a sugar compound classified as a glycosaminoglycan (Amiral et al., 1992, Thrombosis & Haemostasis, 68(1):95-6). Antibodies directed against the PF4/heparin complex can be demonstrated in the blood of patients with HIT, and are associated with the clotting problems seen in these patients. Antibodies to the PF4/heparin complex which have been isolated from humans (Suh et al., 1998, Blood, 91(3):916-22) and mice (Blank et al., 1997, Clin. Exp. Immunol. 108(2):333-9) are polyclonal and heterogeneous since they are derived from the natural polyclonal immune response of an organism to an antigen.
Although autoantibodies against the PF4/heparin complex can be identified in the plasma of more than 90% of patients who develop the clinical syndrome of HIT (Arepally et al., 1995, Am. J. Clin. Pathol. 104:648), these antibodies can also be demonstrated in a significant proportion (15-70%) of asymptomatic patients repetitively exposed to heparin (Amiral et al., 1996, Am. J. Hematol. 52:90; Visentin et al., 1996, J. Lab. Clin. Med. 128:376; Trossaert et al., 1998, Br. J. Haematol. 101:653; Bauer et al., 1997, Circulation 95:1242). The reason why only a subset of immunized patients develop symptomatic disease is unknown (Schmitt et al., 1993, Am. J. Med. Sci. 305:208; Warkentin et al., 1996, Am. J. Med. 101:502; Sheridan et al., 1986, Blood 67:27). The molecular basis for a pathogenic role of antibodies to the PF4/heparin complex has been difficult to establish, since the antibodies are polyclonal and polyspecific.
Heterogeneity in disease expression may, in part, reflect differences in co-morbid factors that predispose to thrombosis, such as atherosclerosis, surgery and vascular trauma (Boshkov et al., 1993, Br. J. Haematol. 84:322; Lee et al., 1998, Thromb. Haemost. 79:50). Others have implicated differences in antibody titer (Suh et al., 1997, Am. J. Hematol. 54:196), affinity (Suh et al., 1998, Blood 91:916), isotype (Amiral et al., 1996, Br. J. Haematol. 92:954), subclass (Suh et al., 1997, Am. J. Hematol. 54:196; Arepally et al., 1997, Blood 89:370), and platelet Fc receptor polymorphism (FcγRIIA-H/R131) (Carlsson et al., 1998, Blood 92:1526) in affected individuals. However, it is clear that such serologic or clinical differences do not permit unambiguous segregation of asymptomatic patients with anti-PF4/heparin complex antibodies from those who develop thrombocytopenia and those who develop thrombocytopenia and thrombosis (Arepally et al., 1997, Blood 89:370; Bachelot-Loza et al., 1998, Thromb. Haemost. 79:523; Herbert et al., 1998, Thromb. Haemost. 80:326).
An additional explanation for the observed heterogeneity in disease expression may lie in the heterogeneity of anti-PF4/heparin complex antibodies themselves. Because PF4 modulates heparin-dependent anti-thrombin (Stem et al., 1985, J. Clin. Invest. 75:272) and protein C co-factor activities (i.e., pro- and anti-coagulant activities {Dudek et al., 1997, C. J. Biol Chem 272:31785}), the effects on these or other coagulant functions of PF4 by antibodies to the PF4/heparin complex remains to be elucidated. Anti-PF4/heparin complex antibodies differ in their antigen specificities, although the responsible determinants have not been clearly delineated (Suh et al., 1997, Am. J. Hematol. 54:196; Ziporen et al., 1998, Blood 92: 3250; Amiral et al., 1996, Blood 88:410; Horsewood et al., 1996, Br. J. Haematol. 95:161). However, the polyclonal nature of the naturally occurring immune response complicates any attempt to determine whether a subset of anti-PF4 antibodies are responsible for thrombosis.
Because of the limited and heterogeneous supply of antibodies from patients with heparin induced thrombocytopenia and/or thrombosis (HIT/HITT), it has been difficult to characterize the pathogenic properties of antibodies to the PF4/heparin complex. Furthermore, in diagnosing HIT/HITT, antibodies to the PF4/heparin complex are needed as a positive control. Until now, tests have relied on the availability of antibodies from patients with HIT/HITT as a “positive control”. Thus, there remains a need in the art for compositions and methods which are useful in the diagnosis and treatment of HIT/HITT, or in the elucidation of the role of antibodies to the PF4/heparin complex in the development of HIT/HITT. The present invention meets this need.